1. Field of the Present Invention
The present invention relates to the solidification of phosphorus pentasulfide by an accretion-type granulation process in order to produce a free-flowing, non-friable product with selective, consistent reactivity.
2. Description of the Prior Art
Accretion-type granulation has been successfully utilized in the production of fertilizers and pharmaceutical products. Accretion granulation is generally conducted by increasing the size of small seed particles through the gradual external addition by coating or fusion of like material in the form of concentrated solutions and/or melts. The granulation process normally utilizes a rotating drum or pan which is designed to form a cascading bed or curtain of undersized seed particles onto which a solution or molten material is sprayed. The important characteristics of a multiple-layered accreted granule are its high hardness, high sphericity, and resistance to form dust.
In U.S. Pat. No. 4,686,115, Majer, improving upon the layering-type granulation process for fertilizers, acknowledges that accretion granulation produces higher hardness, lower tendency to powder, lower tendency to being crushed, and higher sphericity than other granulation methods. In U.S. Pat. No. 4,842,790, Nunnelly describes an improved accretion process that further increases the strength or hardness of the granule and also further decreases its friability. The purpose of Nunnelly's invention is the prevention of fracturing and dust formation and thus the improvement of the bulk handling characteristics.
It is known that granulated fertilizers can be produced by accretion granulation which are free-flowing, anti-caking, non-friable, etc. It is also known that a dustless, granular product will readily mix with a liquid whereas dust and small particles tend to float on the top of the liquid.
Granulation by accretion has never before been used in the production of P.sub.2 S.sub.5, according to the prior art and as indicated by current industry technology.
Prior solidification methods for P.sub.2 S.sub.5 consist of bulk casting, water-cooled screw granulation, and rotary drum flaking. The latter two methods are followed by milling as part of the production process whereas the cast material is generally milled at the user's facility. The rate of cooling P.sub.2 S.sub.5 during the solidification step directly affects the product's crystalline (or amorphous) structure, and thus affects its reactivity. Commercial P.sub.2 S.sub.5 is characterized by the rate at which it will react with alcohols to produce dialkyl thiophosphoric acids; the term "reactivity" as used herein describes this rate of reaction in terms of the amount of temperature rise per minute. Each of these methods results in a different range of reactivity because the cooling rates vary significantly. A gap in the reactivity range exists between screw-granulated P.sub.2 S.sub.5, which typically has a reactivity ranging from 0.degree. to 2.0.degree. C./min, and flaked P.sub.2 S.sub.5, which typically has a reactivity ranging from 3.0.degree. to 7.0.degree. C./min. Users have expressed a need for a reactivity between 2.degree. and 3.degree. C./min, as well as a desire for reactivities greater than 7.degree. C./min.
Several attempts have been made to produce reactivities in this intermediate range and in the extremely high range. Efforts have also been made to predetermine the reactivity, that is, to control the P.sub.2 S.sub.5 solidification or post-solidification process in order to produce specific values of desired reactivity. These attempts have been successful in their intended goals, but their deficiencies are apparent.
U.S. Pat. No. 3,146,069 and U.S. Pat. No. 3,380,808 teach that annealing highly reactive, flaked P.sub.2 S.sub.5 will produce a product that will have a lower, specific reactivity based on the time and temperature of annealing. U.S. Pat. No. 4,419,104 teaches lowering the reactivity of flaked material by controlling the cooling rate of the solidified P.sub.2 S.sub.5 through a temperature range well below the transition zone (i.e., 207.degree. C. to 160.degree. C.). A predetermined, intermediate reactivity P.sub.2 S.sub.5 product can be prepared, according to U.S. Pat. No. 4,248,602, which teaches that blending low reactivity and high reactivity solid P.sub.2 S.sub.5 in quantitative proportions will achieve this end without each component of the mixture reacting successively.
U.S. Pat. No. 3,023,086 teaches that controlling the rate of cooling through the transition zone (i.e., 280.degree. C. to 260.degree. C.) is the most important factor in controlling, or predetermining, reactivity. In order to produce a higher reactivity P.sub.2 S.sub.5, U.S. Pat. No. 4,173,621 applies this concept in a unique way. During drum flaking (or a similar rapid-cooling process), the upper, molten P.sub.2 S.sub.5 -layer is separated immediately from the lower, solidified P.sub.2 S.sub.5 -layer which has cooled extremely rapidly because it is in direct contact with the cooling surface. The upper layer is recycled to the melt and the lower, solidified layer is collected as the highly reactive product. In another attempt to produce highly reactive P.sub.2 S.sub.5, U.S. Pat. No. 3,800,028 describes a method in which P.sub.2 S.sub.5 vapor is rapidly cooled and condensed to a powder product.
While these methods succeed in producing a predetermined reactivity, an intermediate reactivity, an extremely high reactivity, or a combination of these often desirable qualities, none of the methods is flexible enough or wide-reaching enough to encompass all three of these qualities. In addition, none of them result in a product of the inherent consistency of a product produced by accretion. In all three of the prior methods of solidifying P.sub.2 S.sub.5, the portion of material in closest contact with the cooling medium cools much faster and thus has a different reactivity than the more slowly-cooled portion which is insulated from the cooling source. This insulating phenomenon is the primary reason why the resulting reactivity of P.sub.2 S.sub.5 produced by these methods is so unpredictable and the methods thus call for innovative ways to predetermine the reactivity by annealing, post-transition cooling, blending, and removing and recycling layers on the cooling roller.
The present invention teaches a solidification method that does not require any additional steps after the actual solidification step in order to improve the product reactivity. The new process of the invention can also produce very low or very high reactivity P.sub.2 S.sub.5, depending on the controlled process conditions.